This invention relates to a system for damping the lead-lag motion of rotor blades of a helicopter.
A helicopter rotor usually has an even number of rotor blades, typically four in cruciform fashion, wherein the opposed blades are supported by flexible beams or flexbeam to be rotatable about the axis of rotation of the rotor mast. The flexbeam reacts against the centrifugal load between opposite blades and may terminate at a convenient radial position.
In the so-called soft-enplane rotor of a helicopter in which the frequency of natural oscillation in the plane of rotation of the rotor has a value less than the value of the rotational speed of the rotor, there tends to occur a type of aeromechanical instability known as a resonance on ground or resonance in air. This is a kind of self-exciting oscillation which occurs in a case where the frequency of natural oscillation .omega..sub..zeta. in the plane of rotation of the rotor changes in accordance with the rotational speed .OMEGA. of the rotor and the value .vertline.(.omega..sub.70 /.OMEGA.)-1 .vertline. is in agreement with the frequency of natural oscillation of the helicopter body, and if the motion of the rotor in the plane of rotation of the rotor is insufficiently damped, there is a possibility of the helicopter body being placed in an undesirable condition.
In order to eliminate such adverse phenomenon, it has been required for a rotor blade, which is soft in the plane of rotation of the rotor, to be provided with a damping device.
U.S. Pat. No. 4,427,340 to Metzger et al, U.S. Pat. No. 4,244,677 to Noehren et al, and U.S. Pat. No. 4,087,203 to Ferris disclose damping devices for damping the lead-lag motion of a helicopter rotor. According to conventional techniques disclosed in these prior art references, however, pitch change control or regulation of the helicopter rotor must be made by a predetermination of the degree of pitch angle change on the basis of associated phenomenon of the lead-lag motion and the pitch value, which is a difficult matter of the design of the helicopter rotor, and it is also difficult to expect a sufficient stable damping function, as will be described hereinafter. In addition, a complicated structure of the helicopter rotor is required for eliminating the problems described above.